Amplifier circuits used for radio communications, such as those incorporated in mobile phones and the like, are needed to have linearity over a wide range, in order to suppress waveform distortion. Also, in recent years, high-frequency band has come to be used because of the demand for high-speed data communications and the shortage of radio band. As amplifier circuits capable of amplification in high-frequency band, a circuit has been known in which a base-grounded transistor is connected to an emitter-grounded transistor (see, e.g., Behzad Razavi (Japanese translation supervised by Tadahiro Kuroda), “RF Microelectronics,” p. 187, FIG. 6.2(b)).
Conventionally, bipolar transistors have been used in PA (Power Amplifier) drivers and LNAs (Low Noise Amplifiers). To reduce costs of such amplifier circuits, the use of CMOS (Complementary Metal Oxide Semiconductor) process is an effective measure. Generally, however, CMOS transistors with high withstand (or “breakdown”) voltage necessary to provide wide linearity are low in speed, and high-speed transistors have low withstand voltage and fail to provide wide linearity. An amplifier circuit has therefore been proposed in which a high-speed, low-voltage transistor is protected by a low-speed, high-voltage transistor.
FIG. 7 is a circuit diagram illustrating such a conventional amplifier circuit. As illustrated in FIG. 7, the amplifier circuit comprises an amplifying section 101 for amplifying a signal, and a biasing section 102 for applying bias voltages to the amplifying section 101. Transistors M101, M102 and M105 enclosed by a dotted rectangle 111 in FIG. 7 are low-voltage, high-speed NMOS (Negative-channel MOS) transistors, and transistors M103, M104 and M106 enclosed by a dotted rectangle 112 are high-voltage, low-speed NMOS transistors.
In the amplifying section 101, the gate of the transistor M101 receives a bias voltage Vb1 through a resistor R101. Also, the gate of the transistor M101 receives a signal “in” through a capacitor C101. The gate of the transistor M103 receives a bias voltage Vb2.
The signal “in” input to the capacitor C101 is amplified by the transistors M101 and M103. The amplified signal “in” is input to the gate of the transistor M102 through a capacitor C102.
The circuit block comprising the transistors M102 and M104 and resistors R102 and R104 is identical in configuration with the circuit block comprising the transistors M101 and M103 and the resistors R101 and R103. The transistors M102 and M104 receive the bias voltages Vb1 and Vb2, respectively. The signal “in” amplified by the transistors M101 and M103 is further amplified by the transistors M102 and M104 and is output as a signal “out” from the node between the transistor M104 and the resistor R104.
The transistors M101 and M102 constitute a common source circuit, and the transistors M103 and M104 constitute a common gate circuit. The transistors M103 and M104 forming the common gate circuit are connected to the transistors M101 and M102 forming the common source circuit, whereby the gain of the signal “in” is prevented from lowering even in a high-frequency band. Also, the high-voltage, low-speed transistors M103 and M104 are connected to the respective drains of the low-voltage, high-speed transistors M101 and M102 to which the signal “in” is input, whereby the transistors M101 and M102 are protected from a voltage exceeding their withstand voltage, making it possible to implement a high-voltage, high-speed amplifier circuit.
In the biasing section 102, the transistor M105 generates the bias voltage Vb1 applied to the transistors M101 and M102. The transistor M106 generates the bias voltage Vb2 applied to the transistors M103 and M104. The transistors M101, M102 and M105 are fabricated by an identical process so as to have the same characteristics. Similarly, the transistors M103, M104 and M106 are fabricated by an identical process so as to have the same characteristics. Further, the resistors R103, R104 and R105 are fabricated by an identical process so as to have the same characteristics.
It is therefore possible to supply the bias voltages Vb1 and Vb2 as threshold voltages to the transistors M101 and M102 and the transistors M103 and M104, respectively. It is also possible to supply the bias voltages Vb1 and Vb2 that have the same temperature characteristics and the like as the respective threshold voltages of the transistors M101 and M102 and the transistors M103 and M104.
Capacitors C103 and C104 serve to keep the bias voltages Vb1 and Vb2 from varying with voltage fluctuation of a power supply VDD.
FIG. 8 illustrates voltage changes of the amplifier circuit illustrated in FIG. 7. In FIG. 8, waveform W101 indicates a voltage change of the power supply VDD, and waveform W102 indicates a change in the gate voltage of the transistor M102. It is assumed here that in the amplifier circuit of FIG. 7, the transistors M101 and M102 have a withstand voltage of 1.2 V while the transistors M103 and M104 have a withstand voltage of 3.3 V.
When the voltage of the power supply VDD rises as indicated by the waveform W101 in FIG. 8, the bias voltages Vb1 and Vb2 rise with a slope gentler than that of the voltage rise of the power supply VDD, because the capacitors C103 and C104 are connected. Accordingly, while the voltage of the power supply VDD keeps rising, the transistors M101 and M103 experience a period in which their impedances are high.
Since the impedances of the transistors M101 and M103 become high, an electric current flows to the capacitor C102 through the resistor R103 due to the voltage rise of the power supply VDD. Consequently, the gate voltage of the transistor M102 rises beyond the withstand voltage 1.2 V up to about 2.1 V, as indicated by the waveform W102 in FIG. 8.
When the bias voltages Vb1 and Vb2 rise to a level high enough to switch on the respective transistors M101 and M103, a current begins to flow to the transistors M101 and M103. As a result, the gate voltage of the transistor M102 lowers as indicated by the waveform W102 in FIG. 8. The gate voltage of the transistor M102 thereafter stabilizes at around 0.7 V. After the gate voltage of the transistor M102 is stabilized at about 0.7 V which is lower than the withstand voltage, the operation of the amplifier circuit becomes stable.
Thus, in the conventional amplifier circuit, a voltage exceeding the withstand voltage is applied to the transistor when power is applied to the amplifier circuit, giving rise to the problem that degradation of the transistor is caused.